Detection of interstellar propylene in the dark cloud TMC-1 (Forwarded)

Observatoire de Paris
Paris, France

Contact:
Evelyne Roueff
Observatoire de Paris, LUTH, and CNRS
Tel: 33 1 45 07 74 35 Fax: 33 1 45 07 79 71

Maryvonne Gerin
Observatoire de Paris, LERMA, and CNRS
Tel: 33 1 44 32 33 48 Fax: 33 1 44 32 39 92

14 July 2007

Detection of interstellar propylene in the dark cloud TMC-1

The propylene molecule has been detected in the dark interstellar cloud
TMC-1 by a team composed of Spanish, German and French astronomers. Several
rotational transitions have been clearly detected using the IRAM 30m
radiotelescope, located in Pico Veleta (Sierra Nevada, Spain). The propylene
molecule (also called propene), CH2CHCH3, possesses 3 carbon atoms as other
well known and abundant interstellar species (CH3CCH, c-C3H2). and reaches a
similar abundance as those species. However, this molecule was absent from
chemical reaction networks used for modeling the interstellar medium up to
now. This unexpected discovery shows that the exploration of the chemical
diversity of the interstellar medium is far from being complete.

Dense interstellar clouds harbor a rich and exotic chemistry. Many
interstellar molecules have been identified in such clouds, from small and
stable species like carbon monoxide to reactive radicals and long carbon
chains such as C8H or HC11N. Despite the extremely low temperatures (10 K
above zero) and densities (at most a million particules per cubic
centimetre), chemical reactions both in gas and solid phase, enable the
synthesis of the known interstellar molecules. It was considered up to now
that most interstellar species in dark clouds were either reactive or
unsaturated species, in accordance with the detected molecular species. The
presence of saturated molecules was expected in the solid phase, but not in
the gas phase as the gas phase chemical networks led mostly to the
production of reactive and unsaturated species.

The detection of propylene is challenging this hypothesis as this organic
molecules reaches a high abundance in TMC-1, comparable to the abundance of
molecules of similar size and molecular weight (CH3CCH, c-C3H2). The
detection has been more difficult because of the lower dipole moment of the
propylene molecule, leading to weaker rotational lines compared to radicals
with high dipole moments. The excellent sensitivity and good angular
resolution offered by the IRAM 30m telescope have permitted to overcome this
observational bias in the exploration of the interstellar medium chemistry.
The presence of a relatively simple molecule like propylene demonstrates
that the chemical inventory is far from complete and that new discoveries
are awaiting us. Systematic surveys of dark clouds, like the on-going work
of the team led by J. Cernicharo are necessary to continue the exploration
and obtain a more accurate description of the chemistry.

Furthermore, this discovery will lead to significant improvement of the
chemical networks used for modeling the interstellar medium. It is expected
that the respective roles of gas phase and solid phase chemistry in forming
saturated species like propylene will be better evaluated.

Reference:

Discovery of interstellar propylene (CH2CHCH3): missing links in
interstellar gas-phase chemistry
Marcelino N., Cernicharo J., Agundez M., Roueff E. , Gerin M.,
Martin-Pintado J., Mauersberger R., & Thum C.

ApJ Letter in press,
http://arxiv.org/abs/0707.1308

IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/jul07/mol-f1.gif (23KB)]
Spectra of propylene obtained with the 30m telescope of IRAM.

[Figure 2:
http://www.obspm.fr/actual/nouvelle/jul07/mol-f2.gif (17KB)]
The molecule of propylene. The carbon atoms are represented by dark spheres,
while hydrogen atoms by clear grey spheres.